1. Field of the Invention
The present invention relates in general to combustors which are installed in gas turbine engines (hereafter referred to as “gas turbine combustors”) and, more particularly, combustors comprising burners that open into the combustion chamber and mixture injection tubes that inject mixtures of fuel and oxidizer such as air or the like into this combustion chamber.
2. Description of the Related Art
In aero and industrial-gas turbine engines, diffusion flame combustion has conventionally been used. Recently premixed combustion has been used in some industrial gas turbines burning gaseous fuels such as natural gas and is being developed for aero gas turbines. In pre-mixed combustion in gas turbine combustors, mixture of air and fuel is prepared beforehand by supplying fuel into an air stream flowing through a passage connected to the gas turbine combustion chamber. In case the fuel is a liquid, this type of combustion is particularly referred to as pre-mixed and pre-vaporized combustion. In this case, the fuel is not necessarily completely evaporated, but remains in the form of particles. Furthermore, in cases where the amount of air that is mixed with the fuel is considerably larger, than the amount required for complete combustion of the fuel (ordinarily approximately 1.5 times the amount required for complete combustion or more, depending on the conditions such as air temperature), the combustion is referred to as lean pre-mixed (and pre-vaporized) combustion.
The NOx formation rate in combustion shows extremely strong temperature dependence, with NOx being generated in larger amounts at higher temperatures. Since lean pre-mixed combustion is a combustion configuration in which air is present in an excess amount relative to the fuel, the mean combustion temperature is controlled. Furthermore, since the fuel and air are generally well mixed in lean pre-mixed combustion, the formation of local high-temperature regions is excluded, and the combustion temperature is more uniform than in non-premixed combustion. As a result, lean pre-mixed combustion is extremely effective in suppressing NOx formation. Because of the restrictions arising from the heat resistance temperature of turbine materials, the amount of air consumed in combustion in gas turbines is 50% of the total amount of air or less; accordingly, it may be said that lean premixed combustion is a low-NOx combustion technique that is most suited to gas turbines applications in that a large amount of excess air is available.
As the combustion temperature is suppressed by making mixture leaner, the generation of NOx can be suppressed to a greater extent; on the other hand, however, the rate of oxidation of unburned species such as carbon monoxide and fragmented hydrocarbons is also retarded. As a result, the emissions of unburned species tend to increase, and when a certain limit is exceeded, this results in a state in which combustion cannot be sustained at all. This increase in the emissions of unburned species constitutes a drop in combustion efficiency (an increase in fuel consumption), and is not only unwelcome, but is currently impermissible from the standpoint of preventing air pollution.
The mixture ratio of fuel and air is closely related to the combustion gas temperature that governs the formation of NOx. In order to achieve complete combustion and low NOx emissions simultaneously, this mixture ratio must be maintained in a fairly narrow range that includes an optimal value. Attention must be paid to the fact that the optimal value of the mixture ratio is affected not only by the engine operating conditions such as combustion inlet air temperature, the residence time in the combustion region and the like, but also by the temperature and humidity of the atmosphere. In gas turbines, the control of the engine output power and thrust is accomplished by varying the fuel flow rate; accordingly, it is necessary to control the air flow rate in proportion to the fuel flow rate irrespective of power by using a flow rate regulating device, such as a valve or the like in the air passage.
A combustor in which the split of air used for combustion and dilution is controlled by means of butterfly valves is shown in FIG. 14 as an example of a combustor using a flow regulating device. In this gas turbine combustor, it is assumed that the engine is operated at a constant speed. In the gas turbine combustor 130 with controlled air split, extinction of the premixed flame is prevented by means of a diffusion e flame or partially pre-mixed flame of pilot burner 133. Fuel from a fuel nozzle 131 is mixed with air that is forced to swirl by the swirler 134 installed in the entry port of the air passage for the pilot burner 133. The air flows into the combustor from the swirler 134 of the pilot burner 133 that is actuated at the time of engine starting and thereafter, the mixture passages 135 of the main burners 132 that are actuated during operation under a load, dilution air passages 136 with butterfly valves, fixed dilution air holes 137, and cooling holes 139 on the combustion liner 138. Fuel injection holes 140 are disposed in the mixture passages 135. Fuel is injected from the fuel injection holes 140 into and mixed with air, which has been set into a swirling motion by guide vanes or the like, and the mixture is injected into the combustion chamber from the main burners 132. Butterfly valves 141 which modulates the air flow rate are disposed in the premixing air passages 135 and dilution air passages 136. For example, the degree of opening of the butterfly valves 141 can be varied by means of a link mechanism 143 which is connected to an actuator 142 consisting of a servo motor. The link mechanism 143 works so that when butterfly valves 141 for dilution air are substantially fully open, butterfly valves 141 for premixing air are substantially fully closed.
The specific volume of air is much larger than that of fuel. Accordingly, the control of air flow rate requires a mechanical device larger than the control of fuel flow rate. Thus, the manufacturing cost of the air flow control devices is much higher. Another problem of premixing air control is that the flow velocity of the mixture vary in a fairly large range in response to the turn-down of the fuel flow rate (ratio of the maximum flow rate to the minimum flow rate). The upper bound is limited by blow-off and the lower bound is limited by flash-back of flame into the pre-mixing tubes; accordingly, the range in which optimal control can be achieved is generally not wide to cover most of the turn-down required engine operation. Consequently, in cases where the required turn-down is broad, the NOx emissions levels remain high or combustion is incomplete over some range of engine power.
Even if the target for NOx emissions is not set at an extremely low level, the turn-down ratio of lean pre-mixed combustion is considerably narrower than the range required by engine operation.
Another approach for fuel-air ratio control for low NOx emissions over a reasonably wide range of engine operation is the use of a plurality of burners. The number of burners that are being operated is successively increased or decreased in accordance with the output power, i. e., in accordance with the total fuel flow rate. The same principle applies in cases where a large number of burners are divided into several groups, and the number of groups that are being operated is increased or decreased. These method has conventionally been used in many combustor using a diffusion flame or partially pre-mixed flame. In this method, control of the fuel-air ratio can easily be accomplished merely by controlling the fuel flow rate (including switching); accordingly, this method saw immediate practical application in industrial gas turbines using a lean pre-mixed combustion following the introduction of NOx emissions regulations, and has recently used even in aero engine gas turbine combustors.
The structure of a multi-burner type gas-fueled gas turbine engine combustor equipped with eight lean pre-mixed combustion burners surrounding a single diffusion e flame pilot burner is shown in FIG. 15 as one example of a combustor of this type. In the multi-burner type combustor 150 shown in FIG. 15, a plurality of main burners 132 (eight main burners 132), each of which is equipped with a swirler 134 shown in FIG. 14, are disposed at intervals around a pilot burner 133 which is used to maintain premixed flames. A spark plug 152 and dilution air holes 136 which is provided on the downstream side of the spark plug 152 are open in the combustor liner 151 of the combustor 150. In the combustor 150, noting the fact that control of the fuel in the respective main burners 132 is simple, a method in which the number of burners that are lit is successively increased in accordance with engine output power, or a method in which a large number of burners are divided into several groups, and the number of groups that are lit is increased, has been proposed.
This system, unlike mixture ratio control by means of a variable device, the switching of fuel to the burners or burner groups is necessary; accordingly, it is not always possible to maintain all of the burners at the optimal fuel-air ratio, and the following problems regarding fuel arise. In the case of burners or burner groups in which the supply of fresh fuel has been initiated in order to increase the engine output power, the mixture becomes too lean so that even ignition becomes impossible. Or, even if ignition is possible, some burners inevitably pass through a state of incomplete combustion. Furthermore, in cases where only some of the burners are in an operating stage, the flame of these burners or gas in the process of combustion is cooled by the low-temperature air from adjacent burners that are in a non-operating state, so that the emissions of unburned species tends to increase. If an attempt is made to avoid the deleterious effects caused by this interference by increasing the distance between burners, this tends to hinder flame transition between burners (the ignition of burners to which the supply of fuel has been initiated by the flame of adjacent burners that are in operation).
FIG. 16 shows one example of the variation in the NOx emissions concentration and combustion efficiency with respect to the load obtained in an engine test of a multi-burner type combustor 150 shown in FIG. 15. FIG. 16 is a graph which shows the output power on the horizontal axis and the NOx emissions concentration (ppm) and combustion efficiency (η) on the vertical axis in a case where a fixed speed gas turbine equipped with a single flame-maintaining pilot burner and eight identical lean pre-mixed burners was taken as an example. As is shown in FIG. 16, the pilot burner and two of the main burners are operated up to a load of approximately 50%; afterward, the combustor is operated by increasing the number of main burners that are operated two at a time as the load increases, so that all of the main burners are lit at a load of approximately 90% to 100%. In the figures, P+2M indicates operation with the pilot burner and two main burners. Similarly, P+6M indicates operation with the pilot burner and six main burners. The sawtooth variation in the combustion efficiency is attributable to the following: specifically, when operated main burners are added so that the total fuel supply to the main burners is increased in accordance with an increase in the load, the mixture is initially too lean so that fuel is discharged unburned; then, partial combustion eventually begins, so that full combustion finally occurs. The NOx emissions concentration (here, indicated as a value calculated with oxygen concentration of 15%) also varies in sawtooth form with respect to the load; this variation is attributable to the following: namely, in addition to the fact that the emissions of NOx from the main burners is smaller than that from the pilot burner, the generation of NOx is even smaller in a state in which the mixture from the added main burners is partially burned; accordingly, this emission abruptly drops when main burners are added, and then abruptly increases as a result of the temperature rise caused by improvement of the combustion efficiency. In the case of a multi-burner type combustor such as that of the present example, a drop in the combustion efficiency is an unavoidable problem even when the load is high (e. g., 50% or greater). If priority is given to the combustion efficiency, the NOx level abruptly increases. If the number of burners is increased, the drop in the combustion efficiency at the time of addition is correspondingly reduced; however, the fuel flow rate per burner is reduced, so that fine control of the flow rate becomes indispensable.
As was described above, in cases where an attempt is made to realize lean pre-mixed combustion in an engine, control of the flow rate of the air used for combustion by means of a variable device such as valves or the like and fuel staging between multi-burners are indispensable for realizing the low NOx emissions characteristics of lean pre-mixed combustion while maintaining a sufficiently high combustion efficiency across a broader operating range. In regard to this variable device, there are problems in terms of cost and reliability of operation. Furthermore, in regard to fuel staging, the following problems arise: namely, the discharge of unburned components from burners operating at a non-optimal fuel-air ratio increases in the case of partial output power, or else extremely complicated fuel control becomes indispensable.
Accordingly, considering the existence of such problems, the following problems must be solved in a gas turbine combustor: namely, even in the case of a lean mixture which is difficult to ignite, it is necessary to start the combustion reaction of the mixture utilizing high-temperature burned gas so that the amount of NOx generated is reduced by achieving complete combustion and reducing the temperature rise; furthermore, it is necessary to make it possible to control air/fuel ratio during output by performing simplified control of the fuel supply rate without performing complicated control of the flow rate of air for combustion by means of a variable device such as varying the degree of opening of butterfly valves.